1. Field of the Invention
The invention relates generally to cutting elements used on rock bits such as roller cone rock bits, hammer bits, and drag bits. More specifically, the invention relates to drill bits having polycrystalline diamond enhanced cutting elements.
2. Background Art
Roller cone rock bits include a bit body adapted to be coupled to a rotatable drill string and include at least one “cone” that is rotatably mounted to the bit body. Referring to FIG. 1, a roller cone rock bit 10 is shown disposed in a borehole 11. The bit 10 has a body 12 with legs 13 extending generally downward, and a threaded pin end 14 opposite thereto for attachment to a drill string (not shown). Journal shafts 15 are cantilevered from legs 13. Rolling cutters (or roller cones) 16 are rotatably mounted on journal shafts 15. Each roller cone 16 has a plurality of cutting elements 17 mounted thereon. As the body 10 is rotated by rotation of the drill string (not shown), the roller cones 16 rotate over the borehole bottom 18 and maintain the gage of the borehole by rotating against a portion of the borehole sidewall 19. As the roller cone 16 rotates, individual cutting elements 17 are rotated into contact with the formation and then out of contact with the formation.
Hammer bits typically are impacted by a percussion hammer while being rotated against the earth formation being drilled. Referring to FIG. 2, a hammer bit is shown. The hammer bit 20 has a body 22 with a head 24 at one end thereof. The body 22 is received in a hammer (not shown), and the hammer moves the head 24 against the formation to fracture the formation. Cutting elements 26 are mounted in the head 24. Typically the cutting elements 26 are embedded in the drill bit by press fitting or brazing into the bit.
Drag bits are a type of rotary drill bits having no moving parts on them. Referring to FIG. 3, a drag bit is shown. The drag bit 30 has a bit body 32 with a plurality of blades 34 extending from the central longitudinal axis of rotation of the drill bit 36. A plurality of cutting elements 38 are secured on the blades.
Polycrystalline diamond (“PCD”) enhanced inserts and tungsten carbide inserts are two commonly used cutting elements for roller cone rock bits, hammer bits, and drag bits.
Most cutting elements include a substrate of tungsten carbide (WC) based material, which consists of hard particulates of WC, interspersed with a binder component, preferably cobalt, which binds the tungsten carbide particles together. When used in drilling earth formations, the primary contact between the tungsten carbide cutting element and the earth formation being drilled is the outer end of the cutting element. Tungsten carbide cutting elements tend to fail by excessive wear because of their relative softness in comparison to ultrahard materials such as diamond. Thus, it is beneficial to offer this region of the cutting element greater wear protection.
An outer layer that includes diamond particles, such as a polycrystalline diamond (PCD), can provide such improved wear resistance, as compared to the softer tungsten carbide inserts. Such a polycrystalline diamond layer typically includes diamond particles held together by intergranular diamond bonds, which is accomplished by sintering diamond particles together under high pressure/high temperature (HP/HT) conditions in the presence of a diamond solvent catalyst such as cobalt. The attachment of the polycrystalline diamond layer to the tungsten carbide substrate is accomplished simultaneously with the sintering of the PCD material by placing diamond powder adjacent to a WC—Co substrate, and subjecting the diamond powder and the substrate to HP/HT conditions.
The HP/HT conditions used to manufacture the cutting elements result in dissimilar materials being bonded to each other. Because of the different thermal expansion rates between the diamond layer and the carbide, thermal residual stresses are induced on the diamond and substrate layers, and at the interface therebetween after cooling. The residual stress induced on the diamond layer and substrate can often result in insert breakage or delamination under drilling conditions.
To minimize these deleterious effects, the thickness of the polycrystalline diamond layer should be kept at minimum. In prior art cutting elements, the thickness of a polycrystalline diamond layer is typically in the range of 0.006 to 0.010 inches to reduce residual stresses. In fact, the paper “An Analytical Study of the Effects of Multiple Thin Polycrystalline Diamond Coatings on the Enhanced Insert” by Steven W. Peterson (Masters Thesis, Brigham Young University 1995) recommends a maximum polycrystalline diamond layer thickness of 0.008 inches, which was optimized by a finite element analysis-based residual stress study. Increasing the PCD thickness beyond 0.008 inch on enhanced insert products containing transition layers was not recommended because of increased residual stresses. However, this study examined only effects of changes to the design of the PCD and transition layers to lower residual stress, it did not consider the effects of processing.
Various prior art systems have attempted to reduce or remove some of the residual stresses in the cutting element so to avoid bit failure. For example, U.S. Pat. No. 4,694,918 utilizes an outer layer containing polycrystalline diamond with a preferred maximum thickness of approximately 0.005 inches, in conjunction with a transition layer.
Another prior art system, such as that of U.S. Pat. No. 6,199,645, utilizes a polycrystalline diamond layer having a maximum thickness in the critical zone situated between 20 and 80 degrees from the apex of the insert so as to reduce crack propagation due to thermal residual stress. A typical polycrystalline diamond layer maximum thickness of a '645 insert ranges from 0.012 to 0.026 inches.
To reduce residual stresses in inserts that include a polycrystalline diamond layer thickness of greater than 0.030 inches, U.S. Pat. No. 6,651,757 discloses modifying the material properties of the insert such that the polycrystalline diamond has a hardness reduced to between 2000 and 3000 Vickers Hardness Units.
Another known method of reducing residual stress is disclosed in U.S. Pat. No. 5,871,060. The '060 patent discloses the use of textured interfaces to act as a region of intermediate composition between the polycrystalline diamond layer and the carbide substrate.
While these prior art methods are capable of providing PCD cutting elements with improved properties, there exists a need for methods that can provide thicker polycrystalline diamond layers while managing the levels of thermal residual stress.